The International Astronomical Union classifies solar system bodies into three categories: planets, dwarf planets, and small solar system bodies (including comets, asteroids, and meteoroids).
There are eight planets in the solar system with two distinct groupings. The four inner planets
(Mercury, Venus, Earth, and Mars) are rocky and composed of mostly heavy materials. In
contrast, the four outer planets (Jupiter, Saturn, Uranus, and Neptune) are giant planets composed mostly of gaseous elements. Pluto, which was considered a planet until 2006, is not a large gaseous planet: it is much smaller and composed of rock and ice. PlutoÕs composition hints at it being a Kuiper belt object and it was reclassified as a dwarf planet
The Kuiper belt is a region of our solar system that extends beyond the orbit of Neptune.
The solar nebula theory states that the Sun, Earth, and all of the planets in our solar system were
formed within a nebula (a diffuse cloud of dust and gas in outer space), as the gravitational forces between gas and dust particles caused the nebula to contract.